In the last years, an increasing number of wireless communications standards have emerged to provide data and/or voice communication services according to specific needs and quality of service; a mobile user or a mobile communications device may therefore have nowadays the possibility to wirelessly communicate with another user or another communications device using one of the available communications standards which best suites its needs. Usually, the geographic area in which said wireless communications services are available overlaps, and therefore, it is an increasing common situation to find locations in which the mobile user or mobile communications device has the option to select between two or more wireless communications standards for establishing a communications connection or session. Although sometimes two standards for communication may use the same or similar frequencies, the RAT or communications protocol, between the mobile communications device and the radio infrastructure that gives access to a certain communications network, is different, and consequently, the mobile user or mobile communications device has a need to monitor, in a certain location, both a certain number of possible RATs available for establishing a communications connection and a quality of service associated to said RATs.
The above is illustrated in the example of FIG. 1, which shows a typical wireless communications environment comprising two RATs R1 and R2 available for communications service with three wireless communications equipments T1 to T3. Access nodes AN1 and AN5 enable access, each in a certain cell area, according to a first RAT R1, and access nodes AN2 to AN4 provide access, each in a certain cell area, according to a second RAT R2. Therefore, the options for establishing a certain communications connection with the first or the second RAT will depend on the cell area coverage of each access node AN1 to AN5 and on the location of the wireless communications equipment T1 to T3, for example, wireless communications equipment T1 can establish a communications connection with access node AN1 according to the first RAT R1, wireless communications equipment T2 can establish a communications connection with access node AN2 according to the second RAT R2, and wireless communications equipment T3 can establish a communications link with access node AN5 or AN4 according to the first RAT or the second RAT respectively.
Access nodes AN1 to AN5 may be either base stations or access points, depending on the terminology generally used for a certain RAT, such as, for example, WiMAX, WiFi, GSM, UMTS or Bluetooth. If we consider, for example, that the first RAT R1 is a WiMAX access technology and the second RAT R2 being a WiFi access technology, a wireless communications equipment, for example T2, which is located in an area covered only by a WiFi access point AN2, will use a WiFi communications connection with said access point to access the internet; a wireless communications equipment, for example T1, which is located in an area covered only by a WiMAX base station AN1, will use a WiMAX communications connection to access the internet; and a wireless communications equipment, for example T3, which is located in an area covered by both systems WiFi-WiMAX, will, when the corresponding functionality is enabled, select the best radio technology for establishing a communications connection, depending on a certain criterion, e.g. a preferred internet provider, a radio maximizing the throughput or minimizing the power consumption, etc. For the latter use, the wireless communications equipment may be either a bi-mode WiFi-WiMAX equipment or a dual mode WiFi-WiMAX equipment. In bi-mode equipments, only one connection is established at any given time for communication through one or the other RAT and no seamless handover between access nodes of different RATs is possible (the system will first close a certain connection before establishing another connection using a second radio technology, therefore breaking the high level link, e.g. the IP connection). In dual mode equipments, on the other hand, handover from a first RAT to a second RAT is possible without breaking the high level IP connection thanks to the wireless communications equipment capability to establish and maintain two connections at the same time (one with each RAT) during the handover process.
Therefore, there is an increasing need to develop methods and systems which deal with the coexistence of two or more RATs in multi-standard wireless communications environments. Patent application US 2007/0160017 discloses, for example, a system and method for seamlessly roaming in a multi-protocol wireless network environment (e.g. a WiMAX-WiFi environment). The solution proposes a dual-mode mobile station that establishes one communications connection with a certain RAT and, when said RAT is not a preferred one (e.g. the first RAT belongs to a long-range wireless communication protocol), the mobile station checks for the availability of a preferred (e.g. short-range) RAT; and when said preferred RAT is available the mobile station establishes a connection to the access node which offers the preferred communications service. Nevertheless, the above document is silent about how to carry out the method to monitor an access node that offers the preferred communications service. Additionally, no monitoring of the quality of service of the preferred RAT link is done before switching the communications connection to the preferred RAT.
It shall be understood that the most basic approach that could be considered for doing RAT monitoring would be to use some received signal strength indicators (RSSI) that are generally available at the wireless communications equipment's radio frequency (RF) chains, which provide the level of power received on a certain RAT channel. However this would only indicate that there is some activity on that RAT channel, which is not enough in practice, since an activity detected through an RSSI can correspond to a signal transmitted by a mobile device rather than by an access point (AP) or a base station (BS). Therefore, this solution will not be a reliable indicator of the quality of a RAT channel. Besides even if this activity would correspond to that of, e.g. a WiFi AP or of a WiMAX BS, this would not be enough since some information about the AP/BS and the network are necessary to know if a channel can indeed be used (e.g., whether the AP/BS pertains to an authorized internet provider). This information is typically contained inside specific messages transmitted by the BS or by the AP and monitoring would also consist in decoding these messages in addition to synchronizing to the BS or to the AP.
To better illustrate the problem associated with RAT monitoring, we will consider in the following the issue of monitoring a WiFi RAT while the wireless communications equipment has established a communications connection with a WiMAX RAT. WiFi monitoring would comprise successively scanning all the available WiFi channels in order to select the best available channel for establishing a communications connection. WiFi scanning can be done in two ways: either passively or actively, according to section 11.1.3 of IEEE Std. 812.11™-2007, “IEEE Standard for Information Technology —Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, Approved 8 Mar. 2007. Passive WiFi scanning comprises decoding a message periodically sent by the AP, known as beacon, containing all the WiFi cell and network characteristics. By decoding the beacons, the wireless communications equipment knows if a certain channel corresponds to its network provider or home AP and whether it can be used to establish a communications connection with the network. Therefore, at the end of the scanning process, the wireless communications equipment knows which WiFi channels are available and is able to select the best one to establish a connection with the best AP. Since beacons are typically sent every 100 ms, a complete passive scanning will require, considering there are 15 channels, a very long time, about 1.5 s. In order to speed up the process, active WiFi scanning may be used as an alternative. In this case, the wireless communications equipment sends probe messages to test a certain channel and the AP will respond to this request by sending a probe response to the wireless communications equipment containing the information required to establish the communications connection.
In a wireless communications equipment comprising two collocated RF chains, i.e. a RF interface RF1 and RF2 and antenna for each RAT subsystem RS1 and RS2, as illustrated in FIG. 2A, WiFi scanning while the equipment has an established WiMAX connection can be done if the equipment activates the two RF chains simultaneously. The problem with this approach is that, in such a case, a WiMAX signal transmission may prevent active or passive WiFi scanning by disturbing the reception of the WiFi beacon or probe response and, on the other hand, a WiFi signal transmission (when undertaking active WiFi scanning) may harm reception of WiMAX signals. This phenomenon is generally known as “illumination” in the field of cellular communications, wherein a signal transmitted by a base station to a first wireless communications equipment may be completely hidden (illuminated) by the transmission of a second wireless communications equipment that is located very close to the first wireless communications equipment.
In case a passive WiFi scanning is done while the equipment has an established WiMAX connection, the transmission of a WiMAX signal by the wireless communications equipment during the reception of a WiFi beacon would illuminate that beacon. Thus, since passive WiFi scanning requires the decoding of an entire beacon which is periodically sent by the AP every 100 ms and whose duration may vary from 0.1 ms to 2 ms, the probability to decode a beacon would be severely reduced and the overall scanning duration would be largely increased. This solution therefore increases power consumption and is disadvantageous when battery run time is a critical issue. Besides, since a frame duration in WiMAX is typically 5 ms and the beacon periodicity is typically 100 ms, in some cases such an illumination phenomenon would completely prevent the beacon reception since the two durations are integer multiples.
Alternatively, carrying active WiFi scanning while the wireless communications equipment has an established WiMAX connection speeds up the scanning process and reduces power consumption, but on the other hand, WiFi transmission signals disturb or illuminate the established WiMAX connection. Besides, as already indicated for passive WiFi scanning, WiMAX transmission signals disturb the reception of the AP probe response messages.
A possible solution for the above indicated problems could be to properly isolate the two RF chains so that each chain can transmit independently without disturbing the other one. Unfortunately the frequency bands of WiFi and WiMAX are close enough to impose severe isolation constraints between the two RF chains (more than 55 dB of isolation). This would require expensive RF filters and require special care on the design of wireless communications equipments which would considerably increase their unit price.
Additionally, illumination is not only an issue when doing WiFi monitoring in a wireless communications equipment with two RF chains as illustrated in FIG. 2A, since the same phenomenon can occur when WiMAX monitoring is carried while the wireless communications equipment has established a connection with a WiFi RAT. As a consequence, WiMAX scanning can suffer similar problems as the ones indicated for passive or active WiFi scanning.
Furthermore, for a wireless communications equipment comprising only one RF chain, i.e. a single RF interface RF1 and antenna, which is shared by the two RAT subsystems RS1, RS2, as illustrated in FIG. 2B, there is not known solution which allows WiMAX or WiFi scanning without breaking an established active WiFi or WiMAX connection, respectively.